Direct current amplifier



March 10, 1953 w. PARlsoE DIRECT CURRENT 1X1\l1l--I...IIFIERl FiledMarch 1l, 1950 March l0, 1953 w. PARlsoE- 2,631,198,

' DIRECT CURRENTr mvlPLrFnsRy Filed March 11, 195o z'sHEETs--SHEET 2 4'maw. 294 @Wa/M7 Patented Mar. 10, 1953 UNITEDSTATE`S PATENT OFFICEporation of Illinois' Application March I1, 1950, Serial No. 149,156

This invention relates to direct current ampli-v fiers capablev ofdelivering an output current of reversible direction to a load and, moreparticu-- larly, to bridge circuit amplifiers of this character having"electron tubes in at least two, 'se= ries arranged, bridge legs. It isan object of the invention to providev an improved amplifier of thatcharacter.

In one common form, a bridge circuit used as an amplier has four legscontaining impedance devices with a power source connected across twoopposed corners of' the bridge. A load clevice is connected across theother two opposed cornersv and the current therethrough may becontrolled by variation of the impedancevalue of anyone or more of theimpedance devices forming the fourv legs of the bridge. l

, Where a substantial voltage or power ampliiication is required, .avacuum tube is employed as the impedance device for the variableiinpedance leg of the bridge and where greater et@ ficiency andamplification are desired each of the four bridge legs includes a tube,all as well understood in the art. A problem arises, however, in thecontrol o i any two tubes which are arranged in seriesA between thepower source connections because of the fact that such two tubes operateat different voltage levels and, accordingly, cannot be directly drivenby a single D. C. signalvoltage, that is, the signalr voltage cannot beapplied directly across the grid and cathode of both tubes. A

It has previouslybeen proposed to solve this problem by driving thelower voltage level tubes directly and utilizing additional tubes fordriving the higher voltage level tubes. This ar'- rangement issatisfactory operationally but has the obvious disadvantage of requiringtwo extra tubes. The present invention is concerned with the control ofboth the high and low voltage` level tubes without the necessity ofadditional tubes. e

According to one embodiment oi the present invention, the tube operatingat the lower voltage level is driven directly by the signal while theother tube, arranged in series therewithr and operating at a highervoltage level, is driven by the voltage across a biasing resistorarranged inv the plate circuit of the lower voltage level tube. The gridvoltage, and hence the plate current, of the high voltage level tube isthereby varied negatively or oppositely with respect to the platecurrent of the lower voltage tube, that is, when the plate current ofthe lower voltage tube increases, the plate current of the highervoltage tube decreases. The relative values of these twoV plate currentscan be maintained such that: their sumremains constant by selection .of

2 Claims. (Cl. 179-171) a biasing resistor having a proper value withrespect to the characteristics of the driven or high voltage level tube.This relation, or'sorne selected Variation therefrom, is desirable inmost amplier bridge circuits and the method ordea ter-mining the propervalue of theA biasing re-- sistor, for the particular results desired,will be explained subsequently herein. 1

An amplifier incorporating the various fear l turesof the invention iswell adaptedrto use in teleiautographic apparatus since it is capable ofhandling the necessary power while employing standard receiver tubes andsince it isresponsive to direct current signals which iiuctuate atspeeds varying from zero cycles per second'to the frequencies attendingthe most rapid hand-g writing. The amplifier will in fact respond vacIcurately to frequencies inV the audio range er higher. Accordingly, theVarious embodiments of the invention shown in the drawings will'bedescribed below as particularly adapted to that use, but it will beunderstood that the invention isAV not limited thereto.

It is another object of the invention to proivide an improved directcurrent bridge circuit amplifier having a large amplification factor.

It is another'object of theinvention to pro vide an improved directcurrent bridge circuit ampliiier having very stable characteristicsabout the null, or zero load'current region.

It is another object of the invention to pro' vide an improved directVcurrent bridge circuit' amplifier havingV high power eiciency.

It is another object of the invention to provide an improved directcurrent bridge circuit' amplifier having the advantages specified abovewhile being inexpensive to manufacture and reliable in operation.

It is another object offthe invention to providean improved: directcurrent bridgeV circuit amplifier having an. amplifier tube in twolbridge' legs arranged in series between terminals cfa voltage sourceone of these tubes driving the other tube, without the necessity ofadditionalr control tubes,v such that the sum of the plate currents ofthe two tubes is maintained substantially constant. e. y f u Thisinvention, together with further objects and advantages thereof; willbest be und.er-'stoodI by reference to the following description takenconnection with the accompanying drawingsand d its scope willbe pointedoutv in the appended claims.. n c

In the drawingain which like parts are indi-- cated by like referencenumerals?k A Fis, l isa Circuit diagram of tele-autoerapbic apparatileyin'whch a D. C.- bridse amplifier ern-1;

bodying the various features of the invention may be employed toadvantage;

Fig. 2 is a circuit diagram showing a D. C. bridge amplier constructedin accordance with one embodiment of the invention;

Fig. 3 is a circuit diagram similar to Fig. 2 but illustrating anothermethod of introducing the control signal;

Fig. 4 is a circuit diagram illustrating another form of amplifierconstructed in accordance with the invention; and

Fig. 5 is a graph showing typical characteristics of an amplifierconstructed in accordance with the invention.

The tele-autographic apparatus illustratedV in Fig. 1 may be similar tothat described and claimed in the application of Robert Adler, SerialNo. 81,'109, filed March 16, 194:9,V entitled Improvement in Follow-UpApparatus and Systems, issued January 29, 1952, as Patent No. 2,583,720,and assigned to the same assignee as the present invention, and,accordingly, rvvill be described only briey herein.

The apparatus comprises a transmitting station 8 and a receiving station9 connected by a transmission line I0. The transmitting station includesa stylus II with which a message or other form of intelligence may bewritten or drawn on a writing surface I2. The movements of the stylus II are transmitted by a rigid arm I3 to pivotally connected links I4, I5,and I6, the latter two links I5 and I6 being pivotable about aligned,fixed axes I5a and IBa respectively. The link I5 is mechanicallyconnected through an arm I1 -to the movable element of a variableinductance element I8, whereby pivotal movement of the link I5 willcause a variation in the inductance of the coil I8.

Similarly, the link I6 is connected through an arm I9 to a variableinductance coil 20 and I pivotal movement of the link I6 causes a changein the inductance of tha-t coil.

It will be apparent upon inspection of the arm and link arrangement thatmovement of the stylus along the curved X axis indicated on the writingsurface I2 Will leave the link I5 stationary but will cause a pivotalmovement of the link I6 and a consequent change in the inductance of thecoil 20. Similarly, a movement of the stylus II along the curved Y axisindicated on the writing surface I2 will leave the link I6 stationarybut will cause a pivotal movement of the link and a consequent change inthe inductance of coil I8. The link mechanism including the links I3,I4, I5, and I6, shown schematically in Fig. 1, is more fully describedand is claimed in a copending application of Robert Adler, entitledImprovement in Translating Apparatus and Follow-Up Systems, filed April2, 1949, having Serial No. 85,236, issued January 29, 1952 as Patent No.2,583,535 and assigned to the same assignee as the present invention.

The variable inductance coil I8 in combination with a fixed capacitycondenser 2I forms a resonant circuit for controlling the outputfrequency of an oscillator 22, which will be termed herein the Yoscillator, since its output frequency is determined by the position ofthe stylus II along the indicated Y axis on the writing surface I2. Theoutput of the Y oscillator 22 is carried by a conductor 23 to thetransmission line I0 for transmission to the receiver station I I.

Similarly, the variable inductance coil in combination with a fixedcapacity condenser 25 vforms a resonant circuit for controlling thefre-V quency of an X oscillator 26. The output frequency of the Xoscillator, since it is controlled by the variable inductance coil 20,is a function of the position of the stylus II along the indicated Xaxis on the writing surface I2. The output of the X oscillator 26may beconnected by a conductor 21 to the same transmission line I8 Whichcarries the signal from the Y oscillator 22.

At the receiving station, the two signals are separated by the lters 32and 33, the Y filter 32 passing only the Y signal frequency band Whilethe X filter 33 passes'only the X frequency band.

The Y signal passes from the Y filter 32 through an amplifier 34 and alimiter 35 to a discriminator 36.

The discriminator 33 may be of the type described and claimed iniapplication Serial No. 81,709 referred to above, and, accordingly, willnot be described in detail herein. For the purposes of this application,it will be sufcient to say that a direct current signal voltage isproi-v duced thereby Whose magnitude is a function of the frequency ofthe incoming signal from the limiter 35 and of the inductance of a coil36a` forming a part of the discriminator circuit. For every frequencywithin the predetermined band of Y signals, there is a correspondingvalue of inductance of the variable inductance coil 36a which willresult in a zero signal output from the discriminator 36.

The output signal of the discriminator 36 passes through a D. C.amplifier 31 and to the rotor 38 of a D. C. motor 39 Whose eld may beThe motor 39 also drives a receiver stylus 43A through a linkage systemincluding a rigid arm 44 and links 45, 46, and 41. More specifically,the rotor 38 of the motor 39 is mechanically connected to the link 4Iand the link 45, all of these elements being rotatable about a'xed axis46a. When the rotor 38 rotates through a given angle, the link 43 willrotate through the same angle and cause the stylus 43 to move along thecurved Y axis indicated on a receiver station Writing surface 48. k

In order that the operation of the circuit, shown in Fig. 1, may beunderstood to suiiicient degree to permit a full appreciation of thepresent invention, an operation of the apparatus shown in Fig. 1 willnow be described. Let it be assumed that the receiver stylus 43 and thetransmitter stylus II occupy corresponding positions along -theirrespective Y axes. Under these conditions, the variable inductance coil36a has an inductance of such value yand the frequency of the Y signalreceived from the transmitter is of such value that no signal istransmitted from the discriminator 36 to the D. C. amplifier 31Accordingly, the motor 39, the links 4I and 42 and the receiver stylus43 remain stationary.

If now the transmitter vstylus* II is displaced along the Y axis on theWriting surface I2, the inductance of the coil I8 is changed and theout- Y put of the Y oscillator 22 will be of a diiferent frequency thanbefore. Accordingly, the Y signal tion of the transmitter stylus H.Whenthe stylus 43has reached this corresponding position, the inductanceof the coil 36a will have been changed by movement of the links 4l and42 until it is of such Value that at the frequency of the incoming Ysignal the discriminator output signal is zero, all as explained indetail in application Serial No. 81,709 referred to above.

Thus it is seen that the receiver stylus 43 will 'follow the movementsof thc transmitter stylus Il along the corresponding Y axes Thereceiving station includes a circuit for the incoming X signal similarto the Y signal circuit described immediately above. The X signalcircuits and apparatus cause the receiver stylus 43 to assume positionsalong the indicated X axis on the writing surface 48 which correspond tothe positions of the transmitter stylus I l along the indicated X axison the writing surface I2. Since the circuits, the apparatus, and theoperation thereof are identical to the Y signal circuits apparatus andoperation described above, a detail description thereof will not begiven herein.

It will be apparent to those skilled in the art that the signals sentfrom the discriminators to the corresponding D. C. amplifiers are, forpractical reasons, of too small a magnitude to drive the motor 39 of theY signal circuit or the corresponding motor of the X signal circuit.Accordingly, a D. C. amplifier is required for each circuit. Theseamplifiers must be able to handle signal variations of a frequencycorresponding to the rapidity of reversal of` movement of thetransmitter stylus il. In the case of ordinary handwriting, variation inthe D. C. signals fed to the D. C. amplifier normally fall within arange of from cycles per second to approximately cycles per secondAccordingly, where the D. C. amplifier is applied to such a circuit asthat shown in Fig. 1, it must be able to handle these frequencies. Inother applications,` the amplifier may be called upon to respond tofrequencies of a considerably higher range.

In many applications, such as that shown in Fig l, it is also desiredthat the amplifier have a substantial amplification factor in order thata voltage signal of substantialL magnitude may be fed to the motor 39and the stylusf43 be driven rapidly to follow accurately the movementsVof the transmitter stylus Il. l

Still another feature which is desired in the application shown in Fig.1 is high amplifier efficiency. In some applications, the required poweroutput of a D. C. amplifier may be on'the order of milliwatts. In theapplication 0f the D. C. amplifier shown in Fig. 1, however, the desiredpower output is in the order of watts. Accordingly, high efficiency isdesired for Various reasons, perhaps the most important of which is topermit use of standard'and hence low cost circuit elements such as, forexample, amplifier tubes. Another desirable result of high eniciency inthe D. C. amplifier is the reduction in operating power, this being ofvvery substantial significance where the receiving station of thetele-autographic apparatus shown Fig. I is a portable unit `operating onbatteries.

The various D. C. amplifiers or bridge circuits, illustrated in Figs.2', 3, and 4, are well suited for application as the D. C. amplifier 31of the tele-autographic apparatus shown in Fig. l. Any of these threeamplifiers have relatively` high eiiiciencyand can `produce a poweroutput of several watts anda variable-polarity output voltage veryaccurately proportional to a signal voltage which may alternatelyincrease and decrease with a rapidity of from zero cycles per secondv tofrequencies lying in the range of. audio frequencies or even radiofrequencies.

The circuit appearing in Fig. 2 employs four amplifier tubes 1I, 12',13, and 14, each being situated inone of four legs of a bridge circuit.The cathodes 15 and 16 of the tubes 1l and 13', respectively, areconnected together by a conductor 11 which is returned to ground througha pair of resistors 18 and 19. The plate 80 cf the tube 1i is connectedor coupled through a resistor 3f to the cathode 82 of the tube 12.Similarly, the plate 83 of theA tube 13 is connected through `a resistor84 to the cathode 85 of the tube 14. The plate 86 of the tube 12 and theplate 81 of the tube 14 are connected together by a conductor 88 whichin turn is connected to B+, as shown. It is understood, of course, thatB+ vis the positive terminal of a suitable voltage source having itsnegative terminal connect-i ed, in this case, to ground. A load device`83 has its opposite ends connected to the cathodes 82 and or" the tubes12 and 14, respectively.. The B+ voltage is, therefore, applied acrosstwo opposed corners of the bridge while the load device 89 is connectedacross thev other twoopposed corners of the bridge.

When the amplifiers shown in Figs. 2, 3, and 4 are applied totele-autographic apparatussuch as that shown in Fig. l, the load 89 maybe. the motor 39.

The four tubes 1l, 12, 13, and 14 may beof r many types depending on theparticular application of the bridge circuit and more than one tube maybe contained in a single envelope. In the application described above,it is found to be satisfactory to combine the tubes 1l and 13 in asingle envelope, such as a 28D? pentode, with the screen grids 11a and13a connected to B+' as shown, and to combine the two tubes 12 and 14 ina single envelope such as a 6SN'1 or a 12AU7. The screen grids 1Ia and13a are indicatedv as being connected to a voltage source other than theB+ to which the plates of the tubes112r and 14 are connected since thelatter; would' normally be of a higher voltage than should be applied tothe screen grids HaiV and 13a, as isrwellunder.-

. stood in` the art.

A signal S is applied between the grid |00 of the tube 1l and a pointlill intermediate the two resistors 18 and 19. The resistor 18 isprovided-in order to maintain the grid |83` negativewith respect to thecathode 15. If the signal is expected to vary, for example, over a rangeof. seven Volts', from +31@ volts to -31/2 volts, a resistancevaluemight be selected' such that the plate and screen currents for the twotubes 1l and 13, in passing through the resistor, cause a voltageYdrop'of 5 Volts to appear thereacross, resulting in thezcathode 15 ofthe tube 1l being maintainedat av'olt.- age level 5 Volts'above that ofthepoint 10|. Accordingly, the voltage of the grid In!! withrespect tothe cathode 15 would -vary' over. the rangev 5131/2, or *1% to +81/2volts. As will later vbecome apparent,Y the current passing through theresistor .18 will at-allV times bev-substantially constant and in anyevent any small deviations will have no effect other than to make theresponse of the circuit slightly nonlinear; The function of the resistor13 will be explained subsequently. y y Y When the signalY voltage iszero, the voltages of. 'the grids 103 and |32 of the respective tubes-1| and 13, and hence the plate currents of these tubes, are preferablymaintained at equal values. When the voltage of the grid is made morepositive, to increase the current through the tube 1|, it is desired, asis well understood in the art, that an equal but opposite signal beapplied to the grid |02 of the tube 13, that is, the voltage of the grid02 should be made more negative by an equal amount to produce acorresponding decrease in the current passed by the tube 13. In thecircuit shown in Fig. 2, this is accomplished by means of a conventionalvoltage divider.

.A resistor |03 is connected at one end to the grid |02 of the tube 13and at its other end to the plate 00 of the tube 1|, as shown. The grid|02 is connected also through another resistor |04 to ground, as shown.The two resistors |03 and |04 constitute a voltage divider havingimposed thereacross the voltage between the plate 80 of the tube 1| andground. 'W'hen a change in signal voltage makes the grid |00 of the tube1| more positive, the current through the tube 1| increases and theadditional current must pass through the load resi-Stor 89 as willsubsequently become more apparent. The voltage drop across this resistorand the increased voltage drop across the resistor 8| cause the plate 80to become more negative, and since this voltage appears across thevoltage divider resistors |03 and |04 the Yvoltage across the resistor|04 will be smaller and the voltage of the grid |02 will be lower. Thecurrent through the tube 13 will, accordingly, be reduced, and by properselection of the resistors 03 and |04, as is well understood in the art,the Voltage of the grid |02 can be made to vary equally with but inopposition to the voltage of the grid |00 whereby when the plate currentof the tube 1| is increased by one milliampere, the plate current of thetube 13will be decreased by one milliampere and vice versa.

If desired, the voltages of the grids |00 and |02 need not vary equallyand oppositely but the voltage of thegrid |02 can be made to vary inaccordance with an opposite or negative linear function of the voltagechange of the grid |00.

It will' be immediately apparent that the voltage vof the vgrid |02 canapproach but cannot reach ground voltage. Accordingly, in order topermit the voltage of the grid |02 to'be maintained negative withrespect to the cathode 13 (say volts with zero signal voltage assuggested above) the resistor 19 is connected between the point'llll andground. This resistor provides a substantially constant, positive biasto the point |0| and the voltage of the grid |02 can be made to varyabout the voltage level of the point |0|, as does the voltage of thegrid |00. I

With the plate currents of the tubes 1| and 13 varying inversely andpreferably by equal amounts with any given change in signal voltage. asdescribed immediately above, it will be apparentthat any change insignal voltage will cause a change in the current passing through theload 89. However, a greater change in current through the load resistorcan be obtained for a given change in signal voltage by properly varyingthe grid voltages and hence the plate currents of the other two tubes 12and 14. Under balanced conditions, with zero signal voltage, forexample, the plate currents of all four tubes will preferably be equaland no current will pass through the load 89. When the plate current ofthe tube 1 |is increased by a signal variation, the current passed bythetube 12 should be diminf 8 ished while the current carried Vby thetube 14 should be increased as this will make'the bridge circuit moresensistive than if the plate currents of only the tubes 1| and 13 arevaried.

`It may be desirable under certain circumstances to make the platecurrent of the tube 12 equal at all times to the plate current of thetube 13, and the current of the tube 14 equal to that of the tube 1|. Inother cases, it may be desired that the currents of the tubes 12 and 14be made to vary more widely than the currents of Vthe tubes,1| and13'for a given change in signal voltage. It is with the proper controlof the various grids, operating at substantially diiering voltagelevels, that the present invention is primarily concerned.

It is apparent upon inspection of Fig. 2 that the signal S cannot beapplied directly across the grid and cathode of both the tube 1| and thetube 12 since the tube 12 operates at a much higher voltage level thanthe tube 1|. An obvious solution to this problem would be to provideanother tube, controlled by the signal voltage and arranged to drive thegrid |03 of the tube 12.

In the embodiments of the invention illustrated in Figs. 2, 3, and e,the grid of the tube 12 is driven by the tube 1| without the necessityof an additional tube for that purpose. Y

The grid |03 of the tube 12 is connected by a. conductor |04 to theplate 80 of the tube 1|, and, accordingly, its voltage will always bethe same as that of the plate 80. However, the cathode 82 of the tube 12is connected to the plate 80 of the tube 1| through a biasing resistor8|. It will be immediately apparent that with any current passingthrough the tube 1| and hence the biasing resistor 8|, the grid 03 ofthe tube 12 will be negative with respect to the cathode 82 of the sametube. It will also be apparent that as the plate current of the tube 1|increases, the negative bias of the grid |03 becomes greater with aresultant reduction of plate current in the tube 12. Similarly, as theplate Ycurrent of the tube 1| decreases, the negative .bias .of the grid|03 becomes less and the plate current of the tube 12 increases.

The grid |05 of the tube 14 is similarly connected by a conductor |00 tothe plate 83 of the tube 13. The biasing resistor 84 maintains the grid|05 negative with respect to the cathode 85 and as the plate current ofthe tube 13 increases the current in the resistor 84 increases and thegrid |05 is made more negative with respect to the cathode 85, therebyreducing the plate current of the tube 14. Similarly, as the platecurrent of the tube 13 decreases, the plate current of the tube 14increases.

The circuit shown in Fig. 3 is identical to that shown in Fig. 2 withthe exception of the means for applying inverse signals to the grids |00and |02 of the tubes 1| and13, respectively. In Fig. 3 a signal S isapplied across a resistor l|0 having its two endsconnected to the grids|00 and |02 of the tubes i 1| and 13 respectively. The centerV point ofthe resistor ||0 isl connected to ground, as shown. Connecting thecenter point to some point on the bridge circuit of suitable voltagelevelcau'ses the potential of the ends of the resistor ||0, and hencethe grids |00 and |02v to vary equallyand inversely. Connection ofv theresistor to ground as shown in Fig. 3 causes thecathodes A15 and 16 tobe maintained positive with respect to the respective grids |00 and |02by virtue of the voltage drop across the resistor 18 connectngthesecathodes to ground. It will be immediately apparent that a signal of Onepolarity will cause the grid I to become more `positive and the grid |02to become more negative by equal amounts While a signal of the reversepolarity will cause the grid |02 to become more positive and the grid tobecome more negative by equal amounts.

This circuit for applying equal inverse signal voltages to the grids |03and |02 has the advantage of simplicity as compared to the circuit shownin Fig. 2. On the other hand, the arrangement shown in Fig. 3 utilizesonly one-half of the available signal voltage and, accordingly, is notas satisfactory where only a small signal voltage is available.

The actual ohmic values of the resistors 8| and 84 with respect to thecharacteristics of the tubes employed in the circuits shown in Figs. 2and 3 have a controlling influence upon the characteristics of thebridge circuit. Frequently a linear performance of the circuit over amaximum range is desired for various reasons Well recognized in the art.This can be obtained by using resistors 8| and 84 having ohmic valuesequal to Where gm is the transconductance of the respective tubes 12 and14. With resistors 8| and 84 having such values, an increase of say 2milliamperes in the plate current of the tube 1| Will be ac-companied bya decrease'cf 2 milliamperes in the plate current of the tube 12. TheplateV current of the tube 13 in Figs. 2 and 3 Will also be reduced by 2milliarnperes, b-y virtue of the voltage divider ID3-|64 of Fig. 2 orthe resistor |`|0 of Fig. 3, and, as a result of the reduced voltagedrop -across resistor 84, the plate current of the tube 14 will beincreased by the same amount.

The current through the load 89 will be equal to the difference in theplate currents of the tubes 1| and 12, or of the tubes 13 and 14. Theload current is, therefore, a. linear function of the plate current ofthe tube 1| and, accordingly, a linear function of the signal voltage S.As the current through the tubes 1| and 14 continues to increase and thecurrent through the tubes 12 and 13 continues to decrease, the voltagedrop across the load resistor will continue to follow this functionuntil the plate current through the tubes 12 and 13 has reached zero. Atthis point, with a properly designed circuit, the plate current throughthe tubes 1| and 14 will have reached a near maximum value. Any furtherchange in the signal voltage in the same direction will cause relativelylittle change in the current through the load 89 since the currentthrough the tubes 12 and 13 has already become zero.

This characteristic of the circuit may be visualized more clearly in thesolid curve designated gm in Fig. 5, in which the load current I isplotted against the voltage V appearing on the grid Hill of the. tube1|. It will be seen in Fig. 5 that, by making the value of the resistors8| and 84 equal to the reciprocal of the transconductance of therespective tubes 12 and 14, a substantially linear output is obtainedover almost the entire op" erating range of the circuit.

If a linear performance of the amplifier or bridge circuit is notessential, it is possible to 10 obtain an output'which vis greater withrespect t0 the signal voltage'than is available with the re' sistors 8|and 84 being equal to the reciprocal of the transconductance of therespective tubes 12 and 14. This is accomplished by making theseresistors of somewhat higher resistance value. The characteristic curveresulting from such an arrangement is shown as a dotted curve in Fig. 5and is indicated by the expression 1 R gm The steeper portion of rthiscurve, namely, that portion near the indicated vertical coordinate, ismade steep by the fact that a given change in signal voltage produces aproportionate change in the plate current in the tubes 1| and 13, butproduces corresponding changes in the plate currents of the tubes 12 and14 which are greater than the changes appearing in the tubes 1| and 13.More specifically, if the plate current through the tube 1| is increasedby two milliamperes, for example, and the plate current in the tube 13is reduced by two milliamperes, the plate current in the tube 14 may beincreased by three or perhaps four milliamperes, for example, while theplate current through the tube 12 is decreased by three or fourmilliamperes. The net result will, of course, be a current I through theload 89 which is greater 'than that which would have appeared iftheplate currents ofthe tubes 12 and 14 had varied equally with the platecurrent of the tubes 1| and 13, the latter being the case when theresistors 8| and 84 are equal to the reciprocal of the transconductanceof the respective tubes 12 and 14.

The increased response of the tubes 12 and 14, caused by the biasingresistors being greater than the reciprocal of the transconductance ofthe respective tubes 12 and 14, results in the plate current of one ofthe tubes 12 and 14 reaching a maximum desired value and the platecurrent of the other of these tubes becoming zero, before the tubes 1|and 13 have reached the condition of maximum or zero plate current. Thispoint is indicated by the hump in vthe dotted curve in Fig. 5. It Willbe readily understood that as the signal voltage is Varied beyond thevalue correspending to this hump, no substantial change will appear inthe plate currents of the tubes 12 and 14 and, accordingly, the furtherchange in the current through the load 89 is the result of therespective changes in the plate currents in the tubes 1| and 13 only.Accordingly. the curve attens considerably to a slope which is less thanthat of the solid curve shown in Fig. 5.

The use of biasing resistors 8| and 84 having resistance values greaterthan the reciprocal of the transconductance of the corresponding tubes12 and 14 results, then, in a nonlinear response. but causes a greateramplification over at leastpart of the operating range, especially inthe operating range adjacent the zero or neutral signal voltage, Thisarrangement is then more satisfactory Where a linear operating`characteristic is not essential and greater amplification is desiredespecially over the operating range surrounding the zero or neutralsignal voltage.

The curves shown in Fig. 5 are somewhat idealized as applied to thecircuit shown in Fig. y2

since the voltage divider ID3-|04 used to control* the grid |02 of thetube 13 in that circuit results in a nonlinear performance of thecircuit under some conditions.

Another type of bridge circuit is shown in Fig.

4,' this circuit employing Voltage sources in two legs of the bridge andvamplifier tubes in the other two legs. The left-hand portion of thebridge is identical to that appearing in Fig. 1 and the circuit portionshave, accordingly, been assigned the same reference numerals. However,each of the bridge legs making up the right-hand side of theY circuit isa voltage source comprising, in the particular embodiment illustrated, asecondary winding |20 of a transformer, a rectifier |2 and a condenser|22, connected as shown. For most purposes, primary windings |20a,corresponding to the. secondary windings |20, would constitute asingle'winding which may be4 connected to any suitable source ofalternating current having fixed frequency and voltage. With asubstantially constant voltage induced in the windings |20, asubstantially constant D. C. voltage will appear across the condensers|22.

The operation of this circuit will be immediately apparent to thoseskilled in the art in view of the description of Figs. 2 and 3 appearingabove. As the voltage of the grid l of the tube is raised, the platecurrent through the tube 1| and hence through the biasing resistor 8 I.will increase. The increased negative bias of the grid |03 of the tube12 caused by the increased current through the biasing resistor 8|reduces the current-passing through the tube '12. This increase of platecurrent in the tube 'H and the corresponding decrease of plate currentin the tube 12 results, of course, in a current flowing through the load89.

Similarly, the signal S may reduce the plate current of the tube 1| inFig. 3 below the value corresponding to balanced condition and the platecurrent of the tube 'i2 will increase by an equal amount. Current willthen flow from right -to left through load 89.

g The resistance value of the biasing resistor 8| in Fig. 4 may, ofcourse, be made equal to the reciprocal of the .transconductance of thetubey 12, as in the circuits of Figs, 2 and 3, to obtain linearoperation over a maximum range. Also,v the use of a resistor 8| havinggreater resistance` than the value 9m of the tube 12 will result ingreater sensitivity of the circuit, that is, a greater amplificationfactor, but results in a nonlinear output.

While particular embodiments of the invention have been shown, it willbe understood, of course, that the invention is not limited theretosince many modifications may be made, and it is, therefore, contemplatedto cover by the appended claims any such modications as fall within thetrue spirit and scope of the invention.

The invention having thus been described, what is claimed and desired tobe secured by Letters Patent is:

l. A direct current bridge circuit amplier comprising four amplifiertubes, one each in the four legs of said bridge, the cathodes of thefirst and second tubes being connected together and the plates of thethird and fourth said tubes being connected together, a direct currentsource of potential connected between the plates of said third andfourth tubes and the cathodes of said rst and second tubes, the plate ofsaid first tube being'coupled to the cathode of said third tube and theplate of said second tube being coupled to said cathode of said fourthtube, a load connecting the cathodes of said third and fourth tubes, avoltage divider connected at one end tothe plate' of said rst tube andterminating at its other end at a point whose voltage is below that ofthe cathode of said rst tube, a tap on said Voltage divider connected tothe grid of said second tube for controlling the voltage thereof,theposition of said tap being selected whereby the voltage variations ofsaid grid of said second tube are opposite to and equal in amount to thevariations in the grid voltage of said first tube, means for varying thegrid voltage of said third tube in accordance with an opposite linearfunction of the plate current of said first tube including a resistorarranged in the plate circuit of said first tube to carry the platecurrent thereof and an electrical connection between the grid of saidthird tube and the plate of said rst tube, and means for varying thegrid voltage of said fourth tube in accordance with an opposite linearfunction of said second tube including a resistor arranged in the platecircuit of said second tube to carry the plate current thereof and anelectrical connection between the grid of said fourth tube and the plateof said second tube, said resistors each having a resistance valuesubstantially equal to the reciprocal of the transconductance of theadjoining one of said third and fourth tubes.

2. A direct current bridge circuit amplier comprising four ampliertubes, one each in the four legs of said bridge, the cathodes of the rstand second tubes being connected together and the plates of theV thirdand fourth said tubes being connected together, a direct current sourceof potential connected between the plates of said third and fourthtubes'and the cathodes of saidy rst and second tubes, the plate of saidrst tube being coupled to the cathode of said third tube and the plateof saidl second tube being coupled to said cathodev of said fourth tube,a load connecting the cathodes of said third and fourth tubes, a voltagedivider connected at one end to the plate of said first tube andterminating at its other end at a point whose Voltage is below that ofthe cathode of said rst tube, a tap on said voltage divider connected tothe grid of said second tube for controlling the voltage thereof, theposition of `said tap being selected whereby the voltage Variations ofsaid grid of said second tube are opposite to and equal in amount to thevariations in the grid voltage of Said first tube, means for varying thegrid voltf age of vsaid third tube lin accordance with an oppositelinear function of the plate current of said first tube including aresistor arranged in the plate circuit of said first tube to carry theplate current thereof and an Yelectrical connec# tion between the gridof said third tube and the plate of said first tube, and means forvarying the grid voltage of said fourth tube in accordance with anopposite linear function of said second tube including a resistorarranged in the plate circuit of said second tube to carry the platecurrent thereof and an electrical connec tion between the grid of saidfourth tube and the plate of said second tube.

WILBERT PARISOE.

REFERENCES CITED The following references are of record in the le ofthis patent:

UNITED STATES PATENTS Number Name Date 2,310,342 AllZt Feb. 9, 1943'2,329,073 Mitchell el', a1 Sept. '7, 1943 2,424,893 Mansford July 29,1947

